1. Field of the Invention
This invention relates to a method for making a lithium mixed metal compound, more particularly to a method for making a lithium mixed metal compound having an olivine structure and coated with a carbon layer thereon.
2. Description of the Related Art
Lithium-containing transitional metal compounds, such as layered cobalt compounds, layered nickel compounds and spinelle manganese compounds, have been developed for use in positive electrode active materials. However, the cobalt compounds, such as lithium cobalt oxide (LiCoO2), are hardly applied to highly capacitive battery cells due to insufficient resources and poisonous property. The nickel compounds, such as lithium nickel oxide (LiNiO2), are difficult to synthesize and are unstable. While the manganese compounds, such as lithium manganese oxide (LiMnO2), are expected to be suitable for the highly capacitive battery cells because they are relatively economical and safe, they have low capacity, and are unstable and poor in cycle performance. In addition, when the cobalt compounds, nickel compounds and manganese compounds are applied to a battery cell, the initial capacity value of the cell will diminish during the first cycle operation and will further diminish upon every successive cycle of operation.
Another lithium-containing transitional metal compound, olivine lithium ferrous phosphate (LiFePO4), has been considered for use in positive electrode active materials. The lithium ferrous phosphate has good electrochemical properties, good environmental and operational safety, sufficient resources, high specific capacity, cycle performance, and heat stability. Lithium ferrous phosphate has a slight twisted hexagonal close-packed structure that includes a framework consisting of FeO6 octahedrals, LiO6 octahedrals, and PO4 tetrahedrals. In the structure of lithium ferrous phosphate, one FeO6 octahedral is co-sided with two LiO6 octahedrals and one PO4 tetrahedral. However, since the structure of such lithium ferrous phosphate lacks continuous co-sided FeO6 octahedral network, no free electrons can be formed to conduct electricity. In addition, since the PO4 tetrahedrals restrict lattice volume change, insertion and escape of the lithium ions into and from the lattice of lithium ferrous phosphate are adversely affected, thereby significantly decreasing the diffusion rate of lithium ions. The conductivity and ion diffusion rate of lithium ferrous phosphate are decreased, accordingly.
Meanwhile, the smaller the particle size of the lithium ferrous phosphate, the shorter will be the diffusion path of the lithium ions, and the easier will be the insertion and escape of the lithium ions into and from the lattice of lithium ferrous phosphate, which is advantageous to enhance the ion diffusion rate. Besides, addition of conductive materials into the lithium ferrous phosphate is helpful in improving the conductivity of the lithium ferrous phosphate particles. Therefore, it has been proposed heretofore to improve the conductivity of the lithium ferrous phosphate through mixing or synthesizing techniques.
Currently, methods for synthesizing olivine lithium ferrous phosphate include high temperature-solid state reaction, carbothermal reduction, and hydrothermal reaction. For example, U.S. Pat. No. 5,910,382 discloses a method for making olivine compound LiFePO4 powders by preparing intimate mixtures of stoichiometric proportions of Li2CO3 or LiOH.H2O, Fe{CH2COOH}2 and NH4H2PO4.H2O, and heating the mixtures in a non-oxidizing atmosphere at an elevated temperature ranging from 650° C. to 800° C. However, the particle size of the resultant LiFePO4 powders is relatively large, has an uneven distribution, and is not suitable for charge/discharge under a high electrical current. In addition, the ferrous source, i.e., Fe{CH2COOH}2, is expensive, which results in an increase in the manufacturing costs, accordingly.
U.S. Pat. Nos. 6,528,033, 6,716,372, and 6,730,281 disclose methods for making lithium-containing materials by combining an organic material and a mixture containing a lithium compound, a ferric compound and a phosphate compound so that the mixture is mixed with excess quantities of carbon coming from the organic material and so that ferric ions in the mixture are reduced to ferrous ions. The mixture is subsequently heated in a non-oxidizing atmosphere so as to prepare LiFePO4 through carbothermal reduction. However, the methods provided by these prior art patents involve addition of a great amount of organic materials to the mixture, and excess quantities of carbon in LiFePO4 tend to reduce ferrous ions to iron metal and result in loss of specific capacity.
All the aforesaid methods for making LiFePO4 involve solid-state reaction and require long reaction time and a high temperature treatment. The LiFePO4 powders thus formed have a relatively large particle size, a poor ionic conductivity, and a relatively high deteriorating rate in electrochemical properties. In addition, the LiFePO4 powders thus formed are required to be ball-milled due to their large particle size, and the quality of the LiFePO4 powders will deteriorate due to impurity pollution.
In addition, the method for making LiFePO4 through hydrothermal reaction may use soluble ferrous compound, lithium compound, and phosphoric acid as starting materials, so as to control the particle size of LiFePO4. However, hydrothermal reaction is relatively difficult to carry out since it requires to be conducted at a high temperature and a high pressure.
Therefore, there is still a need in the art to provide an economical and simple method for making a lithium mixed metal compound having a relatively small particle size and good conductivity.